Fluid swivels are commonly used in offshore installations to transfer fluids such as gaseous and liquid hydrocarbons between underwater pipelines or wells and a ship. In many applications, as where oil is produced from several wells or signal and service lines are required, a multipass fluid swivel is required. A common multipass fluid swivel includes several swivels stacked on one another, each including a ring-shaped outer wall that rotates about a ring-shaped inner wall, with a toroidal chamber between them. The inner wall of the lowermost swivels have large diameter central holes through which vertical pipes extend to the upper fluid swivels. As a result, the lower fluid swivels require toroidal chambers of large diameter, and correspondingly large diameter seals. Largely because of the need for large diameter lower swivels, common multipass fluid swivels are of large diameter, large weight, and large cost. For example, a common four to eight pass swivel for high pressure fluids (e.g. up to 6,000 psi) for coupling pipes having inside diameters of up to two feet, may weigh up to 200 tons and cost several million dollars. If one of the seals leaks and must be replaced, the stack of swivels must be torn apart, which leads to long and costly delays. In fact, some multipass swivels include extra swivels for use in the event of a seal failure, with the extra swivels resulting in an even larger multipass swivel.
Various attempts have been made to avoid the disadvantages of currently used multipass fluid swivels. One approach has been to use flexible pipes that are wound on and off of spools as the ship weathervanes. U.S. Pat. No. 4,915,416 by Barrett shows one example of this approach. In practice, so called "flexible" hoses are not very flexible, and must be wrapped on large diameter spools, all resulting in large diameter and costly swivels. Another approach which is described in Norwegian patent 885,306 by Paasche et al uses pairs of long flexible hoses extending from each pipe on the ship, with the hose ends being detachably connected to couplings on a nonrotatable turret that is anchored to the sea floor. Where high pressure hydrocarbons must be carried, available flexible hoses are not very flexible, so they must be long to obtain moderate flexing. The long moving hoses require a lot of room and are costly. A multipass fluid swivel arrangement which used fluid swivels of moderate diameter and enabled ready access to the different fluid swivels for maintenance and repairs, as in the event of leaking seals, would be of considerable value.